Sewing machines generally comprise a worktable across which a fabric workpiece is displaced by a workpiece transport device, e.g. dogs emerging from slots in the table or a workpiece clamp, a head overhanging the table and provided with one or more vertically reciprocatable needles, and the associated drive and thread-feed mechanisms designed to allow the needle upon each thrust through the fabric workpiece, to form a stitch of a stitch seam. The vertically reciprocatable needle is generally held in a needle bar which is likewise given an up and down movement, e.g. by a cam, crank or the like.
It is known to terminate a stitch seam, so as to prevent loosening, ravelling or pulling of the sewn thread, with a thickened accumulation of stitches or a locking stitch arrangement at the end of the stitch seam.
A locking terminus for the stitch seam has an advantage over stitch densification in that it is of greater strength although its formation is more complex and hence more expensive.
Stitch densification can be brought about by slowing the advance of the workpiece or merely immobilizing it to allow a multiplicity of stitches to be formed in a limited region. However, to lock the stitch seam it has hitherto been necessary to bring about a change in the direction of advance of the workpiece. For example, a lock formation at the end of a longitudinal stitch seam requires movement of the fabric back and forth while the needle forms the stitches of the lock.
This not only complicates the feed of the workpiece material but, in the case of sewing machines in which the needle is provided on a support for zig-zag movement, also complicates the needle-drive mechanism. Especially in the latter case difficulties have been encountered with the locking of a stitch seam at the end of its formation.